Cold cathode,glow discharge devices



7M ,fm @www WUI"- G R 3 s 45 Z 9 13 3 Si :2; yf; A/ d Q4 DCC- 2, 1969 R. A. DUGDALE ET AL 3,482,133

COLD CATHODE, GLOW DISCHARGE DEVICES Filed oct.. 23, 1967 MLM /DWf/Q 2?/ sv/,Day i L f? T 7 /5 .9 g y x V4 /5 A /f/ y f4 l 27 25' .74 f6 f5 2f f x l y \\\\\\\\\\Q 5 l E L5 T vv,Oz/Mp QM L L' A .l & I fr, .w i y) 1 l, A l!" da) f 4 if f 6 u f "L IUnited States Patent U.S. Cl. 313-210 9 lClaims ABSTRACT OF THE DISCLOSURE A cold cathode glow discharge device having two electrodes mounted in a low pressure enclosure. Potentials maintained on the electrodes generate a glow discharge `and a stream of ions or electrons which can be utilised to melt ceramic material in a Crucible within the enclosure. The electrodes can be shaped to focus the stream of ions.

Background of the invention This invention relates to cold cathode, glow discharge devices, and more particularly to such devices which form electron beam or ion beam sources. The expression cold cathode signifies that the lcathode is non-thermionic.

Such devices have a wide variety of uses, for example, in the heat treatment, welding, melting or zone refinement lof refractory materials. Devices in accordance with the present invention are, however, particularly, but not exclusively, intended for use in the electron beam form for vapourising refractory materials for subsequent condensation on a suitable former or surface, and in the ion beam form for sputtering refractory materials.

Summary -of the invention According to the present invention, a cold cathode, glow discharge device comprises an enclosure, means to maintain the enclosure at a low gas pressure, an electrode arrangement mounted within the enclosure, the electrode arrangement comprising a first electrode including a metallic plate and a second electrode including a metallic mesh arranged generally parallel to but spaced from said plate, the shaping and relative disposition of said plate and mesh being such that said plate and said mesh dene a gap between them, the gap being greatest at their centres and decreases towards their edges, the iirst electrode also including a portion which extends from the edge of said plate towards said mesh, and means to apply potentials to first and second electrodes such that during operation a glow discharge takes place and a stream of charged particles is produced made up from one of the groups of charged particles consisting of ions and electrons.

Preferably said plate is curved so that a focussed stream of electrons or ions is produced.

To produce a stream of electrons the first and second electrodes are arranged to be the cathode and anode respectively, and said portion of the first electrode extends about one third of the distance to said mesh.

To produce a stream of ions the rst and second electrodes are arranged to be the anode and cathode respectively, and said portion of the lirst electrode extends about one half of the distance to said mesh. The stability of the stream of ions is improved by providing a further electrode, 'which may be a ring electrode, on the side of said mesh remote from said plate.

Said plate may be sphercally curved, in which case the stream 'of electrons or ions is in the form of a solid cone, or circular cylindrically curved, in which case the 3,482,133 Patented Dec. 2, 1969 ICC stream of electrons or ions is in the form of a solid wedge.

Description of the drawing A cold cathode, glow discharge device, and a modified form of the device, both in accordance with the present invention, will now be described by way of example with reference to the accompanying drawing `which is partly diagrammatic and partly in section.

Description of the preferred embodiments Referring now to the drawing, the device comprises an enclosure formed by a cylindrical tube 1 of heat resisting glass, closed at the ends by earthed aluminium plates 2 and 3, and supported with its axis vertical.

Within the tube 1 is supported an electrode arrangement comprising a cathode 4 and an anode 5. The cathode 4 includes a circular plate `6 of part spherical shape which is supported at its edges by a right circular cylinder 7. The top of the cylinder 7 is closed by a at plate 8, having apertures 9 therein, which is welded to the bottom of a tube 10. The tube 10 passes up the inside of an insulating tube 11 of glass, alumina or other ceramic to which it is sealed by Araldite (registered trademark) at the top end. The spacing between the tubes 10 and 11 is maintained by a copper wire spacer 12.

The anode 45 includes a circular, metallic mesh 13 of part spherical shape which is supported at its edges by an annulus 1-4 supported in turn by a right circular cylinder 15. The top of the cylinder 1S is closed by a at plate 16 which is welded to the bottom of a tube 17. The tube 17 passes up the outside of the tube 11 to which it is sealed by Araldite at the top end. The spacing between the tubes 11 and 17 is maintained by a copper wire spacer 11S. The spacers 12 and 18 both stop an inch or more from the bottom end of the tube 11.

The plate 6 is formed of thin metal sheet, preferably nickel or tantalum, and the mesh 13 is preferably of tantalum, and preferably has a transmission ratio of or more. The mesh 13 may be formed of 0.010 inch wires spaced at ten wires to the inch. The annulus 14, cylinders 7 and 15, and plates 8 and 16 are thin nickel plate, and the tubes 10 and 17 are thin-walled, stainless steel tubes to reduce the conduction of heat from the cathode 4 and anode 5.

The curvature of the mesh 13 is slightly different from that of the plate 6 so that the gap between them is greatest at their centres and decreases towards their edges. The cylinder 7 extends beyond the plate 6 about one third of the distance to the mesh 13 so that during operation the electric elds associated with the cathode 4 and anode 5 do not show any appreciable edge effect, but are substantially uniform as if the geometry lwere infinite. The spacing between the cylinders 7 and 15, and the plates 8 and 16 is small (1 to 2 mm.) so that no discharge takes place between them during operation, the spacing between the plate 6 and the mesh 13 being much greater (l to 2 cm.).

A power supply 19 is arranged to maintain the potential of the cathode 4 at a value variable over the range 5 to 15 kilovolts negative, and to supply a current of up to l amp. The power supply 19 includes an ammeter and a voltmeter, and means to limit the current (not shown).

Evacuation of the enclosure is by means of a rotary pump 20, connection to the pump 20y being by way of an outlet tube 21 passing through the plate 3. Air enters the enclosure by way of a valve 22 connected to the tube 10 and passes through the apertures 9 into the space between the plate 6 and mesh 13.

The device s operated in the following way. The enclosure is evacuated to a pressure of approximately l to 300 microns Hg, this pressure being maintained adjusting the valve 22 so that the air flow balance the capacity of the pump 20. The cathode 4 is then brought to such a potential that a glow discharge forms, the discharge starting at the centres of the plate 6 and mesh 13 where the spacing is greatest and spreading over the whole area. The discharge has a region adjacent, and parallel to, the cathode 4, the thickness of which depends on the operating conditions. This region represents the cathode fall and most of the applied voltage appears across it. Thus the electric field lines are all perpendicular to the surface of the plate 6 and fast electrons crossing the cathode fall are directed towards the centre of the sphere of which the surface of the plate 6 forms a part. There is thus generated a focussed stream of electrons in the form of a solid cone indicated by the `broken lines 23. Material 24 placed at the point of this cone is therefore heatedmbyghe kineti'ce'nergy of the fast electrons.

The device is iiithded to misdlpaticularly, but not exclusively, to vapourise refractory materials, such as alumina or silica. The material 24 to be vapourised is place in a rotatable crucible 25. After vapourisation the material 24 may be condensed on a suitable former or surface 26. Baffies (not shown) may be provided to reduce contamination of the cathode 4 and anode 5. The surface 26 may, for example, form part of a piece of semiconductor` material on which a layer of refractory material is to be deposited as a step in the production of an electronic device. The former or surface 26 may -be movable for the purpose of securing a required deposition. It may also be arranged that subsequent to deposition the deposited material is annealed by a stream of electrons produced by a further similar device (not shown).

To avoid glow to arc transitions during the initial stages of operation when the cathode 4 is cold, it is desirable to preheat the plate 6 to about 200 C. This may conveniently be achieved by focussing radiant heat onto the plate 6 from a source outside the tube 1, or -by the attachment of an electrical resistance in thermal contact with the plate 6. Alternatively, the device may be started cold, in which case arcing will occur causing the plate 6 to be heated. After a short period, therefore, arcing will cease.

The device may be modified to produce streams of ions in place of electrons by reversing the relative polarities of the potentials applied to the plate 6 and mesh 13, so that the plate 6 becomes part of the anode and the mesh 13 part of the cathode. This form is used when a refractory material is to be sputtered. A more stable stream of ions is produced by the addition of a ring electrode 27, which is maintained at a potential some 100 volts positive with respect to the mesh 13, and so reduces the number of electrons which return to the discharge by producing a space charge sheath on the exterior side of the mesh 13. Other metal parts in the environment should also Abe so biased.

Where streams of ions are to be produced a differnt form of mesh 13 is desirable. This may have a 50% transmission ratio and comprise twenty wires to the inch giving apertures of side 0.035 inch. The form of the mesh 13 (and also the gas used and the gas pressure) affects the potential which it is necessary to apply to the electrode 27 for stabilisation. The smaller the pitch of the mesh the smaller the potential required. i

In the electron beam form of the device the intensity of the stream of electrons can be controlled by the provision of a further electrode which may, for example, be a ring or an apertured plate in a position similar to the electrode 27 of the ion beam form of the device. The control is effected by applying a variable bias of a hundred or so volts positive or negative with respect to the mesh 13.

Although the plate 6 and mesh 13 have been described as part spherical in shape they may alternatively be part right cylindrical in shape, so as to produce a stream of electrons or ions in the form of a solid Wedge coming to a line focus. As a further alternative the plate 6 may be flat so that an unfocussed stream of electrons or ions is produced.

We claim:

1. A cold cathode glow discharge device comprising an enclosure, means to maintain the enclosure at a low gas pressure, an electrode arrangement mounted within said enclosure, the electrode arrangement comprising a first electrode including a metallic plate and a second electrode including a metallic meshy arranged generally parallel to but spaced from said plate, the shaping and relative disposition of said plate and said mesh being such that the gap between them is greatest at their centres and decreases towards their edges, the first electrode also including a portion which extends from the edge of said plate towards said mesh, and means to app1y.rpotentials to the first and second electrpdessuch that during operation a glow'dischargetales place and a stream of charged particles is produced said stream being made up from one of the group of charged particles consisting of ions and electrons.

2. A cold cathode glow discharge device according to claim 1 wherein the charged particles are electrons, the first electrode is adapted to serve as the cathode relative to the second electrode serving as anode, and wherein said portion of the first electrode extends over about one third of the distance to said mesh.

3. A cold cathode glow discharge device according to claim 1 wherein the charged particles are ions, the second electrode is adapted to serve as the cathode relative to the first electrode serving as anode, and wherein said portion of the first electrode extends over about one half of the distance to said mesh.

4. A cold cathode glow discharge device according to claim 1 wherein a further electrode is mounted within said enclosure on the side of said mesh remote from said plate and having means to apply a potential to said further electrode such that during operation the stream of charged particles can be stabilised.

5. A cold cathode glow discharge device according to claim 1 wherein said plate is flat.

6. A cold cathode glow discharge device according to claim 1 wherein said plate is curved so that the stream of charged particles is focussed thereby.

7. A cold cathode glow discharge device according to claim 1 wherein the plate is fabricated from sheet material of one metal of the group consisting of nickel and tantalum.

8 A cold cathode glow discharge device according to claim 1 wherein the mesh is fabricated from tantalum metal.

9. A cold cathode glow discharge device according to claim 1 adapted to vaporise workpieces wherein support means are provided within said enclosure to support a wcrpiece on that side of the mesh remtm'riiwsaid plate.

References Cited UNITED STATES PATENTS 2,570,124 10/1951 Hernqvist 313-63 X 3,156,842 11/1964 McClure 313-63 3,320,475 5/ 1967 Boring 313-231 X JAMES W. LAWRENCE, Primary Examiner RAYMOND F. HOSSFELD, Assistant Examiner Us. c1. X11. 21a-121; 313-63, 216, 23o, 231 

